1. Field of the Invention
The present invention relates to a printing apparatus that creates dots to print an image on a printing medium. More specifically the present invention pertains to a printing apparatus that is capable of arbitrarily creating different types of dots in respective pixels.
2. Discussion of the Background
A diversity of printers have widely been used to print multi-color, multi-tone images as an output device of a computer. One of such printers is an ink jet printer that creates dots with several color inks ejected from a plurality of nozzles provided on a print head, so as to record an image. The ink jet printer generally enables expression of only two tones, that is, a dot-on state and a dot-off state, in each pixel. The ink jet printer accordingly carries out halftone processing, which expresses multiple tones of original print data by a distribution of dots, prior to printing an image.
Multi-valued printers that enable tone expression of greater than two values, the dot-on state and the dot-off state, have been proposed to attain richer tone expression. The multi-valued printers include printers that create dots with different quantities of ink and printers that create a plurality of dots in each pixel in an overlapping manner to express multiple tones. The multi-valued printers ensure a smoother tone expression and high-quality printing.
It is desirable, on the other hand, that the printer attains the desired printing speed and picture quality according to the requirements of the user. In order to meet such requirements, the printer typically has a diversity of print modes, for example, a high-speed print mode and a high-quality print mode. Some multi-valued printers may create different types of dots in respective print modes, in order to attain the desired printing. Such printers mainly use larger-sized dots that enable a solid area to be sufficiently formed in pixels of a relatively low resolution in the high-speed print mode. In the high-quality print mode, on the other hand, such printers mainly use smaller-sized dots corresponding to a relatively high resolution. These dots are selectively used by changing over the driving waveform output to the print head according to the selected print mode.
The recent trend is, however, to further improve picture quality of the printers. This leads to printing at a higher resolution and increases the tone values expressible in each pixel. There is accordingly a significant difference between the printing speed and the picture quality in the high-speed print mode and the printing speed and the picture quality in the high-quality print mode. The user may require a diversity of other print modes, for example an intermediate print mode that carries out printing at an intermediate picture quality and an intermediate printing speed between those of the high-quality print mode and the high-speed print mode.
It is extremely difficult to provide a print head that stably creates dots in response to a diversity of driving waveforms corresponding to respective print modes. Providing the driving waveforms corresponding to the respective print modes leads to an increase in the number of circuits generating the driving waveforms with an increase in the number of print modes. This undesirably raises the manufacturing cost of the printer. The background art printers can not sufficiently attain a required increase in number of print modes, because of these factors. The background art printers can thus not ensure printing that sufficiently fulfills the requirements of the user regarding the printing speed and the picture quality. This problem is found not only in the ink jet printers but in a variety of printers including multi-valued printers and two-valued printers.
An object of the present invention is thus to provide a technique that enables flexible setting of a print mode in a printing apparatus that ensures the expression of different densities in respective pixels according to the print mode.
At least part of the above and the other related objects is attained by a printing apparatus that moves a print head back and forth relative to one axis of a printing medium in response to a driving signal, so as to create a dot in each pixel and print an image on the printing medium. The printing apparatus includes xe2x80x9cnxe2x80x9d output units that periodically output driving signals corresponding to xe2x80x9cnxe2x80x9d different print modes (where n is an integer of not less than 2), in which different dots are created in each pixel and a mixing output unit that periodically uses xe2x80x9cmxe2x80x9d output units (where m is an integer satisfying 2xe2x89xa6mxe2x89xa6n), which are selected out of the xe2x80x9cnxe2x80x9d output units, thereby outputting a specific driving signal that is used in a specific print mode, which is different from any of the xe2x80x9cnxe2x80x9d different print modes.
In the printing apparatus of the present invention, a combination of at least two driving signals output from the xe2x80x9cnxe2x80x9d output units attains (n+1) or greater print modes. One example provides a first driving signal used in a print mode that carries out printing with two variable-size dots L1 and M1 (L1 greater than M1), and a second driving signal used in a different print mode that carries out printing with two variable-size dots L2 and M2 (L2 greater than M2). In the printing apparatus of the present invention, the mixing output unit periodically uses the first driving signal and the second driving signal, so as to attain a print mode using the four variable-size dots L1, L2, M1, and M2.
Here it is assumed that the print mode using the first driving signal is a high-speed print mode. In this case, the two variable-size dots L1 and M1 are relatively large-sized dots, so as to enable a solid area to be formed even in pixels of a relatively low resolution. In order to attain printing at a high speed, it is desirable to raise the driving frequency of the print head. In this print mode, the driving signal corresponding to the other variable-size dots L2 and M2, which are not used for printing, is not output to the print head.
The print mode using the second driving signal is a high-quality print mode. In this case, the two variable-size dots L2 and M2 are relatively small-sized dots to attain printing at a relatively high resolution. There are an extremely large number of pixels in the high-quality printing. In order to implement high-quality printing at a practical speed, it is desirable to keep the driving frequency of the print head at a sufficient level. In this print mode, the driving signal corresponding to the other variable-size dots L1 and M1, which are not used for printing, is not output to the print head.
A specific print mode attaining both an appropriate printing speed and an appropriate picture quality is provided, in addition to the above two print modes. The printing apparatus of the present invention combines the first driving signal with the second driving signal corresponding to the specific print mode. The specific print mode uses the relatively large-sized dots L1 and M1, and thus enables printing at a low resolution equivalent to that in the high-speed print mode. The specific print mode also uses the relatively small-sized dots L2 and M2 as in the high-quality print mode and thus ensures smooth tone expression. The specific print mode accordingly attains an intermediate picture quality and an intermediate printing speed between those of the high-speed print mode and the high-quality print mode.
The printing apparatus of the present invention combines the output units, which are provided to output driving signals corresponding to preset print modes, in the above manner and thereby enables a driving signal to be output corresponding to a different print mode having a different printing speed and a different picture quality, without raising the manufacturing cost of the printing apparatus. The example discussed above addresses a case of setting a third print mode by combining two driving signals. The combination of driving signals is not restricted to the above example. Providing various combinations of three or more driving signals enables a wide range of print modes. The above example addresses a case of using two variable-size dots in each print mode. As long as different dots are used in respective print modes, each print mode may use only one variable-size dot.
In the printing apparatus of the present invention, for example, the mixing output unit carries out printing with xe2x80x9cmxe2x80x9d driving signals. In this case, it is possible to output one driving signal during one pass of the print head (hereinafter referred to as a pass of the main scan) and form each raster line by xe2x80x9cmxe2x80x9d passes of the main scan. It is, however, desirable that the mixing output unit periodically uses the xe2x80x9cmxe2x80x9d output units during one pass of the print head.
The printing apparatus thus constructed may output xe2x80x9cimxe2x80x9d driving signals consecutively in each pixel. This arrangement enables a driving signal corresponding to a print mode to be output during one pass of the print head, and prevents an unnecessary increase in number of back and forth movements of the print head to form each raster line. This results in preferably preventing an extreme decrease in printing speed.
In accordance with one preferable embodiment of the present invention, the mixing output unit causes the xe2x80x9cmxe2x80x9d output units to output driving signals over xe2x80x9cmxe2x80x9d pixels. In this embodiment, the printing apparatus further includes a control unit that controls the mixing output unit to output driving signals to ensure different mappings of the output units to respective pixels during xe2x80x9cmxe2x80x9d passes of the print head, thus enabling xe2x80x9cmxe2x80x9d different driving signals to be output with regard to each pixel.
This arrangement advantageously ensures the overlap effects as discussed below to improve the picture quality. This arrangement also enables the print head to be driven at a driving frequency equivalent to that of the print mode that uses each output unit alone. This advantageously ensures a wide range of combination of the output units without taking into account the restriction according to the driving frequency of the print head.
The advantage of ensuring the overlap effects is discussed below in one example with a combination of two driving signals. The term xe2x80x9coverlapxe2x80x9d means a method of forming each raster line using two or more dot-forming elements in a printing apparatus with a multi-head having a plurality of dot-forming elements. One available technique creates dots in pixels of odd ordinal numbers with a first dot-forming element and dots in pixels of even ordinal numbers with a second dot-forming element. Even if the dot-forming elements have a variation in position of dot formation, using the two or more dot-forming elements to print each raster line preferably prevents a positional misalignment of the whole raster line, thereby improving the picture quality.
In one recording method, the first pass of the main scan forms raster lines in response to the first driving signal, and the second pass of the main scan forms raster lines in response to the second driving signal. In this technique of dot formation, a raster line including only the dots corresponding to the first driving signal is completed by the first pass of the main scan. No dots are accordingly formed on this raster line by the second pass of the main scan. In this area, a resulting printed image has a picture quality equivalent to that of an image printed by one pass of the main scan. Namely there is no overlap effect.
Another recording method alternately outputs the first driving signal and the second driving signal during one pass of the main scan. The first pass of the main scan creates dots in the pixels of odd ordinal numbers in response to the first driving signal and dots in the pixels of even ordinal numbers in response to the second driving signal. The second pass of the main scan, on the contrary, creates dots in the pixel of odd ordinal numbers in response to the second driving signal and dots in the pixels of even ordinal numbers in response to the first driving signal. In a raster line including only the dots corresponding to the first driving signal, dots are created in the pixels of odd ordinal numbers by the first pass of the main scan and in the pixels of even ordinal numbers by the second pass of the main scan. This recording method thus ensures the overlap effects to improve the picture quality. This example addresses a case of combining two driving signals. Similar effects may, however, be exerted by combining three or more driving signals.
The following describes the advantage of ensuring the wide range of combination of output units without taking into account the restriction according to the driving frequency of the print head. The driving frequency of the print head generally has an upper limit according to the mechanism of the print head. In the arrangement of consecutively outputting two different driving signals and causing dots to be formed corresponding to both the two driving signals in each pixel in one pass of the main scan, one applicable method reduces the speed of the main scan according to the number of the driving signals. Another applicable method lowers the driving frequency of each driving signal, such that the consecutive output of the two driving signals satisfies the upper limit. The former method undesirably lowers the printing speed, whereas the latter method unnecessarily reduces the number of different dots in the print mode that uses each driving signal alone, so as to lower the picture quality. In either of these methods, the disadvantage becomes more significant with an increase in the number of driving signals combined.
The technique of the present invention combines two driving signals, such that dots are created in two pixels in response to the respective driving signals. This arrangement makes the driving frequency of the print head in the print mode using the combined driving signals equivalent to the driving frequency in the print mode using each driving signal alone. The technique is thus free from the disadvantages discussed above. This technique enables the number of driving signals combined to increase without any limitation in principle, thus ensuring a wide range of print modes.
In accordance with one preferable application of the printing apparatus of the present invention, the output unit outputs a driving signal of a specific frequency that corresponds to a maximum possible number of dots created in each pixel during one pass of the print head, in response to a driving frequency of the print head.
There is generally a restriction in size of dots possibly created by a single print head. A combination of plural dots created in each pixel extends the expressible density range in each pixel. The use of the specific driving signal corresponding to the maximum possible number of dots created in each pixel ensures an extremely wide range of tone expression using a plurality of dots, and thereby significantly improves the picture quality in any print mode.
In accordance with another preferable application of the printing apparatus of the present invention, at least one of the output units outputs a specific driving signal that enables a plurality of dots including a maximum-sized dot created by the print head to be formed in each pixel.
This arrangement raises the upper limit of the density expressible in each pixel, thus enabling a diversity of variable-size dots to be created and enriching the tone expression as a whole.
The technique of the present invention is actualized by a variety of applications other than the printing apparatus discussed above. For example, in the case that the printing apparatus includes a printer unit that ejects ink to implement printing and a printing controller that drives and controls the printer unit, the technique of the invention may be actualized by the printing controller that generates control data, which are supplied to the printer unit to implement printing according to any of the arrangements discussed above. The present invention is also directed to a method of printing and a method of controlling a printing operation. The technique of the present invention is also actualized by a recording medium, in which a program for driving the printing apparatus discussed above is recorded, as well as the program itself. Available examples of the recording medium include flexible disks, CD-ROMs, magneto-optic discs, IC cards, ROM cartridges, punched cards, prints with barcodes or other codes printed thereon, internal storage devices (memories like a RAM and a ROM), and external storage devices of the computer, and a variety of other computer readable media.